Thin film transistors (TFT) whose channels are formed out of polycrystalline semiconductors have been developed as pixels and pixel drive circuit devices in image display devices such as active matrix type liquid crystal displays or organic/inorganic EL displays, or image sensors. The polycrystalline semiconductor TFTs are advantageous due to their high driving capacity as compared with other drive circuit devices. Thus, peripheral drive circuits can be mounted on one and the same glass substrate together with pixels. As a result, it can be expected to customize circuit specification, to reduce the cost due to simultaneous progress of a process for designing pixels and a process for forming the pixels, or to increase the reliability due to avoidance of mechanical vulnerability in a connection portion between a drive LSI and each pixel. The displays will be also referred to as display devices or image display devices herein.
Polycrystalline semiconductor TFTs for a liquid crystal display are formed on a glass substrate in order to reduce the cost. In a process for forming the TFTs on the glass substrate, the heat resistance temperature of the glass defines the process temperature. An ELA (Excimer Laser Annealing) method for melting and recrystallizing a precursor semiconductor layer with an excimer laser has prevailed as a method for forming a high-quality polycrystalline semiconductor thin film without thermally damaging the glass substrate. The driving capacity of a polycrystalline semiconductor TFT obtained in this method is improved to be 100 or more times as high as that of a TFT whose channel is made of an amorphous semiconductor and which has been used in a background-art liquid crystal display. It is therefore possible to mount a part of circuits such as drivers on the glass substrate. In order to mount higher-performance integrated circuits, however, it is necessary to realize polycrystalline semiconductor TFTs having higher driving capacity.
As a method for forming polycrystalline semiconductor TFTs having higher driving capacity, for example, there is a method in which a semiconductor thin film is scanned relatively with a CW laser beam continuously outputting energy or a pulsed laser beam with a longer time than that in the ELA method, so that the semiconductor thin film is irradiated with an energy beam outputting energy continuously or for a fixed time, as disclosed in Non-Patent Document 1 or Non-Patent Document 2. When the melting time of a semiconductor layer is prolonged, crystal grains are grown in the laser scanning direction. Thus, it is possible to obtain a polycrystalline semiconductor thin film having a large crystal grain size and a uniform grain width and having a flat surface. Hereinafter, growth of crystal grains substantially in the laser scanning direction will be also referred to as lateral growth.
As disclosed in Non-Patent Document 3 or Non-Patent Document 4, repetitious multistage irradiation with a pulsed laser beam with a short time as in the ELA method may be performed with the irradiation position being shifted while scanning with the laser. Thus, crystal grains produced by laser irradiation in the first stage are used as nuclei so that crystals are grown laterally. When the crystals are connected with one another, it is possible to obtain a polycrystalline semiconductor thin film having a large crystal gain size in the laser scanning direction and a uniform grain width.
Polycrystalline semiconductor TFTs manufactured in these background-art methods have driving capacity in their N channels two or three or more times as high as that of polycrystalline semiconductor TFTs manufactured by an excimer laser. Thus, more peripheral drive circuits can be mounted on one and the same glass substrate together with pixels.
Non-Patent Document 1: International Electron Devices Meeting (Washington D.C., 2001) pp. 747-751
Non-Patent Document 2: Society For Information Display International Symposium Digest 2002 pp. 158-161
Non-Patent Document 3: Society For Information Display International Symposium Digest 2004, pp. 868-871
Non-Patent Document 4: IEEE Electron Device Letters vol. 19 1998 pp. 306-308
Patent Document 1: JP-A-2004-22648
In a polycrystalline semiconductor thin film grown laterally as typified by Non-Patent Documents 1 to 4, crystal grain boundaries are formed substantially in parallel with the growing direction. On the other hand, in a polycrystalline semiconductor thin film formed in the ELA method according to the background art, crystal grain boundaries have random directions. The crystal grain boundaries behave as inhibitors of electric conduction. When the current direction is parallel to the laterally growing direction in the former polycrystalline semiconductor thin film, the crystal grain boundaries are no longer primary inhibitors of electric conductions but show a good electric conduction characteristic. However, the crystal grains are not single crystals in a strict sense but contain many crystal defects such as dislocations, stacking defects, point defects, etc. Of these crystal grains, there are some serving as inhibitors of electric conductions.
As crystal grain boundaries often observed in a polycrystalline semiconductor thin film grown laterally, small crystal grain boundary groups each composed of three or more linear crystal grain boundaries arranged in parallel with one another and in an interval not larger than 100 nm are distributed. According to the ELA method, the effect of crystal grain boundaries inhibiting electric conduction is so great that these small crystal grain boundary groups as inhibitors do not manifest themselves. However, in a polycrystalline semiconductor thin film grown laterally, the small crystal grain boundary groups become one of main factors in deterioration of the characteristic of polycrystalline semiconductor TFTs and in increase of variation among the devices. For this reason, the performance of the polycrystalline semiconductor TFTs is not equal to the performance of mono-crystalline semiconductor TFTs, so that circuits the polycrystalline semiconductor TFTs can constitute are limited.
In order to mount drive circuits or peripheral circuits (hereinafter also referred to as peripheral drive circuits) on one and the same glass substrate together with pixels, TFTs having different specifications have to be formed on the same glass substrate together. For example, in a liquid crystal display device, TFTs constituting a pixel circuit thereof have to satisfy properties of a low leak current and a high withstand voltage. On the other hand, TFTs constituting a peripheral drive circuit for processing an external input signal and converting the signal into an analog signal are requested to have a high on-current, a low threshold value, a low variation and a steep rising edge. There is no polycrystalline semiconductor thin film for supplying TFTs satisfying these specifications simultaneously. It is therefore necessary to produce polycrystalline semiconductor thin films having different film qualities on one and the same glass substrate.
As a method in which polycrystalline semiconductor thin films having difference film qualities are produced on one and the same glass substrate, there is a method including a first irradiation step in which a silicon material is irradiated with a CW laser and a second irradiation step in which at least a part of the silicon material is irradiated with a pulsed laser beam in an oxygen-containing atmosphere, wherein the film surface is oxidized and formed into a thin film by abrasion in the first and second irradiation steps, as disclosed in Patent Document 1. According to this method, it is supposed that two kinds of polycrystalline semiconductor thin films having different film thicknesses can be produced on one and the same glass substrate. In this method, however, the polycrystalline semiconductor thin film obtained in the first irradiation step is not reformed so that the inhibitors of electric conduction as described above cannot be removed. It is also difficult to control the film thickness by abrasion. In addition, in order to obtain the effect of reduction in leak current according to the aforementioned Patent Document 1, it is more effective to shield an active layer from backlight by use of a light shielding film or the like than to reduce the film thickness of the polycrystalline semiconductor thin film so as to control the effect.